Regex to Add Thousand Separators to Numbers

Add Thousand Separators with Regex

For display purposes, large numbers are easier to read with separators every three digits. The classic regex uses a lookahead to insert a comma after every digit that has a multiple-of-three count of digits remaining.

Comma Separator

str.replace(/\B(?=(\d{3})+(?!\d))/g, ",")

Breakdown:

• \B matches a position that is not a word boundary (so the leading edge does not get a comma)
• (?=(\d{3})+(?!\d)) looks ahead and ensures the remaining digits are an exact multiple of three

Tested Examples

Input	Output
"1234"	"1,234"
"1234567"	"1,234,567"
"123"	"123"
"1234.56"	"1,234.56" (only integer part is touched)
"-9876543"	"-9,876,543"

Explicit Integer-Part Targeting

When the number has a decimal portion, restrict the replace to the integer part:

function format(n) {
  const [int, dec] = String(n).split(".");
  return int.replace(/\B(?=(\d{3})+(?!\d))/g, ",") + (dec ? "." + dec : "");
}

Different Separators

European convention uses dots for thousands and commas for decimals:

str.replace(/\B(?=(\d{3})+(?!\d))/g, ".")

For Indian numbering (12,34,567), the grouping is 3-2-2:

str.replace(/(\d)(?=(\d\d)+\d$)/g, "$1,")

12345678 becomes 1,23,45,678.

Practical Recommendation

For locale-aware formatting, use Intl.NumberFormat:

new Intl.NumberFormat("en-US").format(1234567.89)  // "1,234,567.89"
new Intl.NumberFormat("de-DE").format(1234567.89)  // "1.234.567,89"

The regex approach is useful when you need to format numbers embedded in larger strings (e.g., log lines or templates) without parsing them as numbers first.